test: Demonstrate leaking of display for cram test

<!-- ps-id: bca52dae-c074-4edc-8022-073c2c7b551d -->

Signed-off-by: Ali Caglayan <alizter@gmail.com>

test/blackbox-tests/test-cases/cram/git-diff-fail.t

@@ -0,0 +1,26 @@
+We demonstrate the leaking of the display of internal processes when running
+cram tests.
+
+First we make a cram test:
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.7)
+  > EOF
+
+  $ cat > mytest.t << EOF
+  >   $ echo A
+  >   B
+  > EOF
+
+We need to avoid the special treatment of the test when INSIDE_DUNE is set:
+  $ unset INSIDE_DUNE
+
+We get nonsense internal output in the display:
+
+  $ dune build --root=. --diff-command="exit 123" --display=short @runtest 2>&1 >/dev/null | head -n2
+  File "mytest.t", line 1, characters 0-0:
+            sh (internal) (exit 1)
+
+The rest of this message will look like this:
+/nix/store/4xw8n979xpivdc46a9ndcvyhwgif00hz-bash-5.1-p16/bin/sh: line 1: exit: too many arguments
+[1]